1. Field of the Invention
The present invention relates to a steam-replacement deaeration apparatus and method for use in bag packaging. More particularly, the present invention relates to a steam-replacement deaeration apparatus and method capable of appropriately controlling the temperature of steam to be injected. The present invention further relates to a steam-replacement deaeration apparatus and method capable of appropriately controlling the steam temperature and also capable of rapidly heating up steam injected from a nozzle to an appropriate temperature for steam-replacement deaeration, thereby reducing the standby time of a bag packaging operation.
2. Discussion of Related Art
In automatic bag packaging of food products or the like, steam-replacement deaeration is performed in which steam is blown into a bag after it has been filled with an article to be packaged to replace the air in the bag with the steam, thereby effecting deaeration. The temperature of steam injected into the bag is important because it influences the contraction of the internal volume of the bag after cooling and also a treatment carried out after the deaeration, e.g. a sterilization treatment.
Conventional steam-replacement deaeration apparatus used in bag packaging are generally arranged as follows. Steam supplied from a steam supply source is heated in a heating device incorporating a cartridge heater, and the heated steam is sent to a nozzle through a steam supply passage. The steam temperature at the outlet of the heating device is measured, and a signal representing the outlet steam temperature is feedbacked to control ON/OFF of the heater so that the outlet steam temperature reaches a target value. The surface temperature of the heating device is also measured to prevent the heater from overheating when no steam is flowing therethrough, thereby preventing a failure of the heating device.
In the conventional steam-replacement deaeration apparatus, steam from the heating device is sent to the nozzle through the steam supply passage. Therefore, a temperature drop naturally occurs during the delivery of the steam. Consequently, the temperature of steam actually injected from the nozzle differs from the steam temperature at the outlet of the heating device. Further, even if the temperature at the heating device outlet is the same, the temperature drop differs according to the change of the ambient temperature, or between when the apparatus has started its operation and when it is in steady-state operation, resulting in a difference in the temperature of steam injected from the nozzle. For this reason, deaeration failure occurs frequently, and the operator needs to make an adjustment every time such failure occurs.
Japanese Patent Application Post-Exam Publication No. Hei 7-86012 discloses an arrangement for controlling a heater in automatic packaging using cup-shaped semirigid containers in which steam and an inert gas are successively blown into each container. In the disclosed arrangement, a coil heater is wound around a steam pipe, through which steam passes, as far as near the end of a steam branch pipe, and a heat-insulating material is wound on the coil heater. Steam passing through the steam pipe is heated by the coil heater, and the temperature of steam is measured with a thermometer attached sidewardly to a lower end outlet of the steam branch pipe from which steam is injected. A signal representing the measured temperature is sent to a controller to control the heater. In this regard, bag packaging process using bag-shaped containers needs to insert the steam injection nozzle into a bag in order to obtain a satisfactory deaerated state and therefore cannot employ an arrangement as disclosed in the above-mentioned patent document, wherein a pipe through which steam passes is per se wound with a coil heater and a heat-insulating material to heat steam passing therethrough, and a thermometer is attached sidewardly to a lower end outlet of the pipe.